Horses being used on the cancelled television series Luck - which starred Dustin Hoffman - were drugged and sick, according to an animal rights worker who has filed a lawsuit.

Barbara Casey is suing the show's producers, the television network HBO, and the American Humane Association (AHA) claiming the animals on the show were grossly mistreated.

Miss Casey, who was director of production in the AHA's film and television unit, was overseeing the conditions of the animals being used on the series.

In court papers filed in the Los Angeles Superior Court, she claims she was fired after complaining they were severely mistreated.

She claims the animals were underweight, sick and drugged for the television series focusing on the horse racing industry.

The series was cancelled in March after four horses died while in production and the animal rights organisation PETA sharply criticised the treatment of the horses.

Image:Four horses died during the prodution of the cancelled television show Luck

In her lawsuit, Ms Casey claims that the AHA, HBO and the show's producers, Stewart Productions, covered up the abuse months before the series was cancelled.

The court papers allege: "AHA bowed to political and financial pressure and refused to report the production defendants' conduct to the authorities."

They said: "AHA instructed plaintiff not to report such conduct. AHA engaged in efforts to conceal and cover up the production defendants' criminal activities."

The association declined to comment, citing the pending lawsuit.

HBO told The Hollywood Reporter: "We took every precaution to ensure that our horses were treated humanely and with the utmost care, exceeding every safeguard of all protocols and guidelines required of the production."